This invention relates to communication switching, and in particular, to the synchronization of the transmission of control information.
communications systems, it is often necessary to exchange control information between a central control unit and distributed units. In general, there are two main methods of performing this exchange of control information. The first method is to utilize a controller, normally a computer, in the central control unit and in each of the distributed control units. When the main control unit needs to modify the control information being utilized in a distributed unit, the computer in the main control unit simply sends the changes in the control information to the distributed unit. A computer in the distributed unit then properly modifies control tables with the changed information. The problem with this technique is in the reliability of software that is utilized to control the computers. Software problems can cause significant problems within the communications system.
The second method is often referred to as synchronous control. In the synchronous control method, the main controller continuously retransmits all of the control information to the distributed units. The interval at which the information is repeated is referred to as the frame interval. Synchronous control is utilized in certain telecommunications systems. In addition, certain television systems transmit textual information to television sets as Teletext using synchronous control. The distributed units also use synchronized control to communicate their control information to the main controller. Synchronous control has many advantages one of which is the fact that data being distributed among the distributed units is also being transmitted on the basis of the frame interval. Thus, the required timing signals are already available. Synchronous control does have a serious problem. Within synchronous control, the value of the control data, as well as its precise position within the frame, determines how the data will affect the unit receiving this control information. The problem occurs when timing or data errors affect the content of a frame. To prevent these problems, error-checking protocols are utilized over the entire content of the frame to assure that the frame has been correctly received by the receiving unit before the receiving unit utilizes the control information contained in the frame. The problem with this prior art solution is that in telecommunications systems the amount of data contained in each individual frame may be large resulting in a large amount of storage being necessary to allow the complete checking of the frame before its utilization.
The foregoing problems are solved, and a technical advance is achieved, by an apparatus and method in which control data is communicated utilizing a multi-frame format that consists of individual frames each having a number of rows and columns. As each row is received from a transmitting unit, the data and position of the row within a frame is verified by use of a checksum and is immediately utilized by the receiving unit. Advantageously, this reduces the amount of storage that must be provided by the receiving unit for the reception of control information to that of a single row. In addition to verifying the row identification, the identity of each frame within the multi-frame is also verified before an individual row is utilized. Advantageously, in a first embodiment of the invention, the operation of generating the checksum is seeded with the current row number. Advantageously, in a second embodiment of the invention, the operation of generating the checksum is seeded with the result of an Exclusive OR operation of the current row number and frame number. Advantageously, the multi-frame protocol used for the communication of control information is similar to the STM protocol.
Other and further aspects of the present invention will become apparent during the course of the following description by reference to the accompanying drawing.